I'm just asking if anybody can help me add a pickup to a valve radio.
Some sets have a "Gram" input.
If I wanted to add one of these to a radio, obviously the shielded or negative terminal would go to ground (chassis) but where in the circuit should I wire the + terminal? Do I need to couple it via a capacitor? If so, of what value would be best?

Magnetic coupling with an audio transformer is probably better if connecting it to modern stuff.
What radio? and do you have schematics? Many models have something in the same lineup with a phono input.
The radio front end, and probably the AGC is switched out of the audio as well when the phono is selected.

Depends a bit on the type of pickup and its output level. With the older crystal pickups, it could be connected to the top end of the volume control, with a "radio / gram" switch to select either the audio from the detector, or the signal from the pickup. With newer magnetic pickups, the level is probably too low for this connection and a pre-amplifier may be needed.
A cap of about .01 to .047 would probably be a good idea to block any DC on the volume control pot resulting from biasing or AGC.

What sort of pickup are you looking at?
Harold

An assumption that can get you into strife is to assume that one terminal is ground. I have a Philips here that has "back bias" and a magnetic pick up. That pickup is "floating" and has to be DC blocked or, if grounded it will short out the bias to every valve in the set.

A transformer applied without knowledge of the OP of the source & PU of the valve set may cause destruction or damage to both radios.

Marc

It's an old ac=dc set from around 1934, the chassis is negative, I have another old 30's set, with the pickup terminals, which I wired to a mono "male" plug, with an adapter using a mono "female" wired to a stereo "male" plug, which I then use to plug into my computer and can then play music, either from my files or the internet, you-tube etc, and it comes out of the radio sounding really nice.
As the radio with the pickup input is destined to be given to my mum, I wanted to see if I could adapt my other radio to be able to play music from other sources in the same way.

"Its an old AC DC set". That means that it cannot be earthed & the chassis has a 50:50 chance of being alive to Mains. If the chassis is currently Neutral you may be lucky. If its Active you better pray you have RCD's & I doubt that they will trip after a rectifier.

Those sets are a death trap.

Marc

My earlier post assumed you wanted to attach a pickup to play vinyl records. It is an entirely different ball game to attach some digital device as impedances and signal levels will certainly be different.
Also, the prospect of attaching such a device to a possibly live chassis, (see Marc's comment above), would scare me away from any thought of doing it.
Harold

Maybe I'll just find another radio with an existing "Gram" input.
I certainly don't want a death trap\
Vinyl records would be on the cards too
with the one I plan to give to my mum, the sound through the gram pickup is good.
My laptop is compatible, the gain if anything a little low, but full tone comes through...
Mobile phones only drive it to a bare whisper though.

Mobile phones and tablets think you are using earphones when you plug them directly into anything else, that is why the volume is low so as to protect your ears. The best way to get around this is to connect them through a bluetooth receiver which also means that if anything goes awry then you are only risking a 20 to 30 dollar gadget rather than a 500 to 900 dollar one.

Aha, I shall investigate that option.

Be aware that if you are connecting an vinyl record player to your amp you will need RIAA equalisation.
See:-

https://en.wikipedia.org/wiki/RIAA_equalisation